1. Field of the Invention
This invention relates to valves and more particularly to temperature and chemical resistant, high pressure gate valves.
2. Description of the Prior Art
A non-rising stem gate valve with separable seats and automatic plastic sealed gate-seat and seat-body interfaces is disclosed in U.S. Pat. No. 2,433,638--Volpin. A similarly sealed valve of the rising stem, balanced type intended for high pressure use is disclosed in U.S. Pat. No. 3,538,938--Volpin and 3,696,831--Fowler et al. In the latter two patents, stem packing compressed by packing glands is employed for the stem seals.
In recent years, a few wells have been drilled in very high pressure sour gas fields. These wells have bottom hole pressures to 22,000 psi (152 MPa) and bottom hole temperatures up to 385.degree. Fahrenheit (196.degree. Centrigrade). The well fluids contain methane, some CO.sub.2, salt water, and high percentage of H.sub.2 S, such as 27 to 46 percent H.sub.2 S for example. Lower pressure fields with high percentages of H.sub.2 S are also being produced. Several references have indicated that the standard oilfield valve elastomer stem packings which have worked well with fluids containing small amounts of H.sub.2 S and at temperatures less than 250.degree. Fahrenheit (121.degree. Centrigrade) will deteriorate and leak after only a short period in an elevated temperature and H.sub.2 S environment. See, for example, Hamby, T. W. and Tuttle, R. N., "Deep High-Pressure Sour Gas Wells--An Industry Challenge," 1975 Annual Meeting, Division of Production, American Petroleum Institute and Hamby, T. W., Broussard, L. P., and Taylor, D. B., "Producing Mississippi's Deep High Pressure Sour Gas," Paper No. SPE 5604, Society of Petroleum Engineers of American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., 1975.
The types of valve stem packing used to date in oilfield valves have included molded packing rings, such as the chevron packing, compression packing with an adjustable packing gland, and plastic injected type packing between elastomer seals. To control the effects of the sour gas on these seals, elastomer materials, such as fluoroplastics and asbestos compounds, have been tried with some success. However, the combined effects of the higher pressure and elevated temperature have caused failures of seals made from these new materials as well.
The primary design requirement for a high pressure valve stem packing is to seal, bubble-tight, gas with a high percentage of H.sub.2 S at 25,000 psi (172 MPa) working pressure at 300.degree. Fahrenheit (149.degree. Centrigrade) with hydrostatic test pressure at, for example, 37,500 psi (259 MPa). Additional design requirements are low friction, low maintenance, and a long service life. The preferred design for the valve stuffing box would allow application of the seal to balanced or unbalanced stem, rising or non-rising stem valves and would be as short as possible. The stem packing cycle life is important and should be relatively high, such as, for example, 300 cycles without leakage. A cycle is defined as the stem movement to first open a gate valve and then close the valve. The 300-cycle number is, for example, based on operation of a valve once per week for approximately six years. Designs which did not rely on extended neck bonnets or air cooling fins to lower the packing temperature are also desirable.
A valve known as a "Graygate" appears to employ a plurality of seals, which may have anti-extrusion washers disposed on the top and bottom surfaces of each seal, around the valve stem and in a bonnet pocket, but the valve is not actuated by pressures arising from pressuring the plastic sandwich seal. See, for example, U.S. Pat. No. 3,990,679 to Boitnott.
It is believed that the valve of the present invention can be used without leakage in environments of high H.sub.2 S content with higher pressure and over a wider temperature range than the valves discussed above.